1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of the Related Art
An electric power steering (EPS) system using a motor as a driving source has been proposed as one type of power steering systems for vehicles. The EPS system is characterized by a high degree of flexibility in the layout thereof and a small amount of energy consumption. In recent years, therefore, the electric power steering system has been increasingly employed in a wide variety of vehicle types, from compact vehicles to large vehicles.
In the EPS system, numerous abnormality determining operations (diagnostics) are performed in information processing equipment (such as a microcomputer) that performs power assist control of the EPS system. For example, the microcomputer includes CPU and memories (RAM and ROM), and various types of electronic circuits (such as A/D converters). Upon start-up of the microcomputer (i.e., when the ignition key is turned on), an initial check is conducted so as to check if a memory that provides a storage space for a program under execution and its work data is normal. See, for example, Japanese Patent Application Publication No. 2006-331086 (JP-A-2006-331086). After start-up, too, it is checked if the microcomputer and various electronic circuits under control of the microcomputer function normally. If any abnormality is detected in the abnormality determination process, a fail-safe action or measure is immediately taken so as to ensure high reliability and safety.
In recent years, the EPS system is configured to perform various compensation controls in an attempt to achieve more excellent steering feeling, resulting in an increase in the capacity or storage of the memory and an increase in the time required for the initial check. Also, the function checks after start-up are conducted by a monitoring microcomputer provided independently of a main microcomputer that implements power assist control. In this case, the main microcomputer has to carry out test computations sent from the monitoring microcomputer, in real time. Furthermore, the provision of new monitoring circuits (abnormality determination circuits), including the monitoring microcomputer, makes it necessary for the main microcomputer to check if the monitoring circuits function normally. Thus, the processing power required to perform the abnormality determination process keeps growing or increasing, which is a cause of increase in the manufacturing cost.
In addition, the provision of a large number of monitoring circuits leads to a significant increase in the failure rate due to an increase in the number of elements of the system. Also, since abnormality determinations are made over a wide range of phenomena, even a trivial phenomenon, which would hardly affect the operation of the EPS system, may be determined as an abnormality. If such an abnormality is counted as a failure, the failure rate may be further raised or increased, as one example of problems caused by the provision of the monitoring circuits. In this respect, too, there is still room for improvement.